There are many applications in modern day electronic equipment for amplifier circuits having a high degree of common mode rejection which diminishes the creation of unwanted signal components at the output terminals thereof. More specifically, such amplifiers are often desired for implementation in integrated circuit form for utilization in automotive applications. One such application relating to engine fuel control requires a stable, unity gain amplifier having a high input impedance and a high degree of common mode rejection. Such amplifiers must provide an output voltage having precisely the same magnitude as an analog input voltage occurring across a storage capacitor.
One prior art monolithic amplifier circuit configuration includes an NPN differential amplifier having two differential NPN transistors with a current source connected to the common emitters thereof. A further current source is connected to the collector of the output differential transistor and a further NPN transistor is connected between the further current source and the control electrode of the output differential transistor. This amplifier configuration meets the requirements of the previous paragraph except for the requirement relating to common mode rejection. More specifically, the internal impedances associated with the current sources and with the input differential transistor tend to cause undesired imbalance in the signals of the circuit. As the magnitude of the input voltage applied to the base of the input differential transistor increases, the voltage at the common emitter point of the differential transistor also increases. This produces a change in the magnitude of the current of the differential amplifier current source. Also, the driving voltage produces different changes in the magnitudes of the currents flowing through the input differential transistor and in the output current source. Since these changes in current magnitudes are not equal, the emitter current of the differential pair tends to undesirably change as a function of the change in the magnitude of the input voltage. As a result, the differential or common mode voltage across the differential amplifier changes as a function of the input voltage magnitude change thereby causing the magnitude of the output voltage of the amplifier to not precisely follow the magnitude of the analog input voltage as required.
Another prior art circuit utilizes a differential amplifier including inside and outside pairs of PNP transistors. A current source is connected to the emitters of the inside pair of PNP transistors. A differential-to-single ended converter including a diode and a NPN transistor is connected between the collectors of the inside pair of PNP transistors. An output NPN transistor is connected between the converter and the output differential amplifier transistor. The differential-to-single ended converter tends to force the emitter currents of the inside pair of differential transistors to be equal independent of the value of the current of the current source. The inside pair of differential transistors maintains balance in the outside pair of PNP transistors. Thus, the PNP differential amplifier provides a high degree of common mode rejection.
However, the foregoing prior art PNP differential amplifier tends to have relatively high input currents because of the low betas of the PNP transistors, which can undesirably load the driving circuit. This problem is particularly troublesome if the circuit is fabricated in monolithic integrated circuit form. Such loading can be disadvantageous, for example, if the driving circuit includes a storage capacitor having an analog voltage of a precise magnitude stored thereon since the loading will change the magnitude of the analog voltage. Furthermore, this PNP amplifier tends to limit the excursion of the output voltage thereof because of the base-to-emitter junction drops of the transistors connected between its output terminal and the positive power supply conductor. However, the most serious disadvantage is that this amplifier has two stages of gain which tend to oscillate when the amplifier is connected in a unity gain configuration. Therefore, large compensation capacitors are required to prevent oscillation. Such capacitors on the monolithic integrated circuit are costly or if provided externally will require additional pin-outs which increase the costs associated with the manufacture and use of the integrated circuit.